Delayed pulse radar navigation apparatus



March 20, 1951 F. C. THOMPSON Filed July 25, 1947 4 Sheets-Sheet 1MASTER MULTIVIBRATOR Flg'l TRANSMITTER TR 7 5 AND R. "j

RECEIVER MonuLAToR A S-AWTOOTH PULSE GENERATOR\ GENERATORR\IULTIVIBRATOR SG TPC M V r M ROTARY ROL TRANSFORMER v CONT TIMING M Ty CIRCUIT OLAMPING CIRCUIT 7 V PARA- CC PHASE V AMPLI'? 1 -1PA| PAZSIGNALS -PHASE Y AMPLIFIER I"? L X i x PA3 HEIGHT PPI TUBE H PLANPOSITION lNDlCATOR Ff gjggizgson Attorney March 20, 1951 F. c. THOMPSONDELAYED PULSE RADAR NAVIGATION APPARATUS 4 Sheets-Sheet 2 Filed July 25,1947 Fig.2

Inventor B F. C. Thompson Attorney March'ZO, 1951 F. c. THOMPSON,545,754

DELAYED PULSE RADAR NAVIGATION APPARATUS Filed July 25, 1947 4Sheets-Sheet 3 FORMING CKT.

PULSE Inventor F. 0. Thompson 377% W01} Attorney March 20, 1951 c,THOMPSON 2,545,754

DELAYED PULSE RADAR NAVIGATION APPARATUS Filed July 25, 1947 4Sheets-Sheet 4 Fig.6

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Attorney Patented Mar. 20, 1951 DELAYED PULSE RADAR NAVIGATION APPARATUSFrederick Charles Thompson, Danbury, England Application July 25, 1947,Serial No. 763,604 In Great Britain July 20, 1945 Section 1, Public Law690, August 8, 1946 Patent expires July 20, 1965 6 Claims. 1

This invention relates to improvements in navigating apparatus of thekind in which the terrain being navigated is scanned by a pulsemodulatedradio beam and echoes of the pulses are displayed on a cathode ray tubein a manner such as to present a map-like picture of said terrain. Acathode ray tube employed in this manner is known as a plan positionindicator.

The invention is concerned with reducing a particular form of distortionof the display which occurs when such apparatus is airborne. an exampleof such an apparatus functions as follows:

The transmitting aerial, which usually also acts as the receivingaerial, is adapted to emit a narrow beam which at any instant covers asmall sector of the terrain in question. The cathode ray tube isprovided with a radial time base which is triggered by a transmittedpulse, or vice-versa, and as successive echoes of the pulse are receivedthey are caused to brighten the trace from the centre of the tubeoutwards. By causing the aerial and the radial time base to rotate insynchronism and by repeating the process of transmitting a pulse anddisplaying the echoes along the radial trace for each small sector, acomplete Such an apparatus installed at or near ground level gives atrue scale picture of the surrounding terrain on the screen of thecathode ray tube. When airborne, however, distortion is introduced dueto the time taken for the pulses to reach the ground and return asechoes. For example, in the case of a ground installation, echoes fromobjects immediately surrounding the aerial are received more or lessinstantaneously and since the radial scan of the cathode ray tube timebase starts as the transmitter fires these echoes appear substantiallyat the centre of the screen. In an airborne installation no echoes arereceived until the radial scan has moved outwardly from the centre ofthe screen a distance proportional to the height of the aircraft andconsequently features on the ground vertically below the aircraft appearnot at the centre of the tube but on a circle which increases in size asthe height of the aircraft increases, leaving a dead area at the centreof the display. The size of the area and the distortion introducedbecome considerable when the aircraft reaches a height which is nolonger negligible compared with the distance represented by a radius onthe display. For instance, if the radius of the screen represents tenmiles on the ground and the aircraft is flying at a height of fivemiles, the ground return ring, as the said circle is called, is half theradius of the screen.

The object of the present invention is to eliminate the ground returnring in apparatus of the kind set forth and to this end according to theinvention, means are provided for synchronising the instant each radialsweep starts from the centre of the tube with the instant of receipt ofthe first echo from said terrain of a transmitted pulse associated withsaid sweep.

The start of the time base may be said to be delayed from the instant oftransmission of a pulse until the instant of receipt of the first echothereof.

According to another aspect of the invention means are provided forinitiating each radial time base sweep at an instant after thetransmission of a respective pulse determined by the height of theapparatus above said terrain.

The timing of the first echo and the timing of the start of the radialtime base may be compared on the screen of a second cathode ray tube andmeans may be provided for adjusting the timing of the start of the timebase to obtain coincidence of the indication.

The comparison of the timing of the first echo and of the start of thetime base may be effected for example, by displaying the said echo as adeflection of a line trace on said second tube in one direction and thestart of the time base as a deflection in the opposite direction.

The deflection or marker pulse which indicates the start of the timebase is advantageously used to trigger the radial time base generatingcircuit.

In order to enable the nature of the invention and. in what manner thesame is to be performed to be more readily understood reference isdirected to the accompanying drawings in which- Figure l is a schematicblock diagram illustrating one practical embodiment of the presentinvention applied to the relevant circuits of a navigating apparatus ofthe kind set forth.

Figure 2 is a series of wave form diagrams and Figures 3 to 6 arecircuit diagrams of the said embodiment.

In Figure l, the plan position indicator PPI is the cathode ray tubeadapted to give the maplike display in question. A height tube H is afurther cathode ray tube provided with a linear time base'and adapted toindicate echo time, i. e. range, by lateral deflections of the trace.The echo time with which this invention is principally 3 concerned isthat of the first ground echo, which is a function of the height of theaircraft.

The radial time base of the PPI tube originates from a sawtoothgenerator SG controlled by a master multi-vibrator MMV.. The sawtoothwaveform is fed to 'thejrotor f a rdtary transformer RT which hasstators in quadrature and these outputs are applied through paraphaseamplifiers PA! and PAZ respectively to the X and Y deflector plates ofthe PPI tube. v Theresult is a radial time base rotating insynchronism'with the rotor of the rotarytransformer 3T, and, if theaerial device A is mounted onor'tllrnsywith the rotor shaft of thetransformer, the beam emitted by the aerial will also rotate insynchronism with the radial time base. f V

The receiver which detects and amplifies the echo signals and modulatesthe intensity of the radial trace is combined with the transmitter toform. .A.m li :ribrat ;l Y q nirp em uiater. Mot the transmitte -linemult vibrator Ml/Tisj ,trig eredwbya timing pulse gener- .ator. 'IEG.which is.contro led,,b lie sawmi generator SG so that the multi-vibratorvrv is synchronised with the master multi vibrator m rtt omnomr llin themo u e er .thegmulti-vibrator Mll isadapted to delivera synchronisinpulse. eve timehe. tra sm tte .fires, to a he ht ma kentimins ircui MThis circuitis. adapted int oduc a he ght marktp s g n eonusai usync rqns n pu s b a variable time. dete m ned y thafiett n 9i contro1-C-.-:he..h ieht ar erpu s j fedtq the left ha d Xnlat o t ehei ht ubefigndnmeals as a efle tion at the le tqi theyer i a trace. The time base forthe verticaltrace of the. heighttube isderived iron; the sawtoothgener'ator; SG, and suitably arnplified by a paraphase .a'mplifier :PAQ.By altering; the control C, this l defie'ction a can be made to move upor down the trace.

.. -Theejcho, signals from the receiverflthesame .signals asare usedt mu ate he, beam intensi- .ety." of, the .RPI tube) ar edto. he. i t. h

.taircraft. I If then the control C is adjusted until the deflection a,is lev l. witllth IQW -S defl c io .b the delay introduced :bythe heightmarker timingcircuit HMT also becomes a function of the aircraft height.

. tial do es not drop immediately. but'afterashort inte rval determinedby" the eandenser "C 2 which hasffirst "to charge negatively through"the ca'thr ode-screen impedance of the valve. The slip- 4 an outputwaveform substantially as shown in Fig. 2a. This waveform is applied tothe timing pulse generator 'IPG and multivibrator MV at the terminal Ashown in Fig. 3. Valve VI is a timing valve which develops a negativepip at 'its an ode whencarried into conduction by the sawtooth waveapplied to its grid and the timing of the pipe relatively to thesawtooth can be adjusted by means of the variable resistance VRI which,in conjunction with the resistance Rl,

"controls the positive D. C. potential of the grid about which thesawtooth swings. Fig. 2b shows tred, about a small D. C. level butactually out ofi at 0 volts due to grid current. Anode cur-'rentfiowsasthe grid is raised above cut-off iay the sawtooth wave andthe anode potential falls "asshown in Fig. 2c. (Cut-off is only justbelow 0 volts due to the low anode potential and consequent short gridbase.) The screen potenpressoris normally at o vo lts, due to the diodeDj, butis tied tethes'c'reenby the'condensr C2 andtherefor e is driven'negative when the screen I potential" falls. This" immediately cuts"o'if the anod current and the iano d potentialrises sharply to itsnormal value. Thejanodewave- V form is thus a series of negative pipsoccur'iiig at' a point on a Sawtooth waveform determined .by rar bier teq B!- in f as'the present invention is concer'ned the adjustment biasvvhich develops on thegrid condenser due i9 i c rrent.

of VRl is purely arbitrary. :Thene'gative autorough' thei'girid stopperis subacted y the bleeder- R2 consfien iel iie est d 1 9.3 e lli lfiivid 3 iesmo the de V? whi .ic e i J73 constitutes the multi-vibrator Mvwhi'ch "is p The negati ve lgoing pips from the anode of Vl free runningat a repetition frequency slightly 4 less than that of thedmastermultivibrator'lfl/l v (Fig. l) but which locks to the master frequency.underjthe control of said negative pips. A nega- The height markerpulse isfalsoifed tova clamp- 'tive going 20 sec rectangularp-ulse isobtained from-the anode of V3 and is used to develop a trigger pulsewhich fires the transmitter on the back edge -of the 2% sec. pulse. v

.. A' similar, 'butpositive-goingQZO sec. ;pul se (Fig- 2d) isdeliveredfrom the-cathode of V3 at terminal Band is; used to develop theheight marker previously referredjo. TerminalB is "connected to terminalBi :01" 7 EigA 'which is the height marker timing circuit HMT of Fig. 1.v

, Valve V6 is the actual height markertiming valve, and is a-diode-triodewith a common cathode. The ZOJL sec; pulse isa'ppIied-tothe primary Of: a; pulse transformer Ti and-'appears atthe secondary asanegatiVegOin pulse with an os- '1 cillation due to ringing at its backedge (-Eig.

. and PAZare inoperative and notra ceappears on the PPL'tube until theclamping circuit CC is ,triggerediby the height marker pulse. Therefore,by maintaining the adjustment of thecontrol C so that the height markera is alwaysglevel with I the first. ground return b; the j radial time"base of the. PPI tube starts at the instant" the 'filSt ground echo isreceived and the object-of the invention, namely, the elimination of theground return ring, is achieved, y a, V I Certain of the circuits; willnow be described in, a detail. The a ioe h ne a e";

any suitable known form) is adapted to deliver 2e). This 'latter' pulseis applied to the cathode ,Of -V4. Normall the grid-thediode anode; andthe cathode are returnedto aspositive potential determined by the heightcontrol variable re- 7 sistance -VR.2 andtheanode-to' a higher positive:potential. The triode is therefore normally cutoff;by grid current biasbut the-diode is "just conducting and condenser C3'is-charged. Theleadingedge of the applied pulse drives the cathode negative to the gridwhich does not follow. owing to the long time constant of '(34R5 there rPas esr d. in -Iii. r fan The triode current, the condenser C3discharges, and the anode voltage falls. At the end of the pulse periodthe cathode rises to the potential of the grid, the condition ofauto-bias is restored and anode current ceases but the anode voltagedoes not immediately rise again to the maximum value determined by thelimiter valve V5 since condenser C3 is discharged and requires to becharged through the anode load R6. The delay is determined by the normaldifference in potential between the anode and the cathode. That of theanode is fixed by V5 and that of the cathode by the variable resistanceVR2 which therefore constitutes the control C described with referenceto Fig. 1. This maximum delay occurs when the slider of VRZ is set forminimum potential and must be at least equal to the echo time of thefirst ground return when the aircraft is at its ceiling. Fig. 2 showsthe waveform at the anode of V4when VR2 is set for zero height (fulllines), medium height (broken lines) and maximum height (chain lines).It will be noticed that the oscillations at the back edge of the pulsein Fig. 2e is not transmitted to the anode. This is due to the action ofthe diode portion which prevents the lower plate of C3 from rising abovethe potential of the slider of VRZ.

Valves V6 and V1 form a cathode coupled flipflop. V6 grid is subject tothe anode potential of V4. The arrangement is such that a rectangularnegative-going pulse (Fig. 2h) appears at the anode of V1 and a similar,but positive-going, pulse (Fig. 29) appears at the anode of V6. Thesepulses commence on the leading edge of the pulse from the anode of V4(Fig. 2/) and ceases when thepotential of said anode raises V6 gridabove cut-ofl. The cut-off level of V6 grid depends on the currentpassed by V'l through the common cathode load R1 which current iscontrolled by the potential on V1 grid as derived from the variableresistance VR3. This variable potential is arranged to be a few voltsbelow the limited maximum anode potential of V4 and provides a pre-setzero control for initial calibration purposes.

A pulse forming line LC in the anode circuit 'of V6 converts the squarewave (Fig. 29) to positive and negative pips (Fig. 21) which areinverted by transformer T2 and applied to the negatively biased grid ofa cathode follower V8. The bias is just suflicient to hold the valvecut-off until a positive going signal is applied. Consequently only thepositive going pip of the waveform shown in Fig. 21' appears at thecathode of V8 and this (Fig. 210) is fed through a 2 sec. delay lineLICI to the output terminal C. The dela is introduced to compensate forthe lag which occurs in practice between the back edge of the 20 ,u.sec. pulse of multi-vibrator MV and the firing of the transmitter. Theoutput from terminal C is amplified and applied to the left hand X plateof the height tube H (Fig. 1) where it appears as a deflection a to theleft of the vertical time base trace. By adjusting the variableresistance VRZ (control C) the deflection is caused to move up and downthe trace and when deflection a is level with the lowest deflection bthe timing of the height marker is synchronous with the receipt of thefirst ground return from each transmitted pulse.

The height marker from terminal C is used to trigger the radial timebase of the PPI tube (Fig. 1), by means of a relaxation-relay stagecomprising valves V9 and VII] (Fig. 5) and a series of diode clamps VII, W2, and VB each of which comprises two diode sections which forconvenience are illustrated separately and notated VI la,

VI lb, etc. Valves V9 to Vl3 thus constitute the clamping circuit CC ofFig. 1. The relaxationrelay state (valves V9 and VI 0) is described incopending United States patent application Serial No. 762,375, in thename of F. C. Williams.

Figure 6 is a circuit diagram of the paraphase amplifiers PM and PAZ forthe radial time base of the PPI tube shown in block form in Fig. l. Thequadrature sawtooth outputs from the rotary transformer RT are appliedto the primaries of transformers T3 and T4 the secondaries of which areconnected to the grids of valves Vl and W5 which are respectivelycathode coupled to valves VIB and Vi! to constitute two paraphasesamplifiers connected to the deflector plates of the cathode ray tubePPI.

As hereinafter stated, the invention eliminates the distortion due tothe delay in receiving the first ground echoes after the transmission ofa pulse but if a linear radial time base is employed there will still bedistortion since the echo time is a function of slant distance and notdistance measured in the plane of the terrain. A circuit arrangementsuitable for correcting such distortion is advantageously embodied ineach of the grid circuits of valves W4 and Vl5 shown in Fig. 6. Theyconsist of resistance capacity networks BC I and RC2 connected betweenthe grids of the valves VM and W5 and the common line 2 of the secondarywindings of transformers T3 and T4. These networks RC! and RC2 requirea.

constant charging voltage and therefore the sawtooth outputs from thetransformers T3 and T4 are differentiated by means of seriesresistancecondenser combinations RC3 and RC5 to give substantiallysquare waveforms for application to the networks ROI and RC2.

The action of the circuit of Fig. 5 in clamping and unclamping thepotentials at the three points. I, 2, and 3, indicated by the arrows inFig. 6,.

will now be described. Normally, until a height marker pulse is fed toterminal Cl (Fig. 5), thesuppressor grid of V9 is at earth potential,the anode is at a limited positive potential determined by the diode VHJand potential divider RSRE, the screen is at a positive potential lessthan the anode as determined by a potential divider comprising variableresistances VRE and VRE, the control grid is at anode potential, and thecathode is at some positive potential. The potential divider comprisingVD5 and VRE is connected across positive and negative lines and thestrapped cathodes of the diodes Vii, W2, and VI3 are connected to anegative point on said potential divider. Under these stable conditionscathode current raises the cathode potential sufficiently to cut offanode current by means. of suppressor grid bias, since the suppressorgrid, is earthed through transformer T5, and all the cathode current iscoming from the screen which is at its minimum potential. The strappedanodes of the diodes are also arranged to be at the same negativepotential as the strapped cathodes by connection to a negative point onthe potential divider comprising resistances RH],

RH and VRI. The three mutually connected anodes and cathodes 0f thediodes are connected, as indicated by the arrows l, 2 and 3, to the gridcircuits of valves VM and V|5 (Fig. 6) and to the common line .2respectively. This common line 2 is held at the same negative potentialas the strapped anodes and cathodes of the diodes VI l, V12 and VI3 bymeans of a potential divider comprising resistances RI: and VR-B acrossthe hegatiye lir e and earth.- The grids 9f V4 9 fidd y 5 ar'inpt,'thefce'f qr :r'ise or fl ll from the lriegative pqtent'i al of the'strapped' 'ahbdes and (is-t ese of the diddes pe' au s dthe' T'W-is'ediode cohdi qtien oeurs' iri' one or'pthey {lalf '91 the diodes.Conseqiienfly," Wfien; the c iycuit vs JT-l-iliis in' its stago le cqnd-itiim a ing square ave developed iil eith'ei' resistai lde'fctlfideiiser Q m-bi'na-ti' RC3 foiQRCfi, as'the i'esultgf laI i isin'g sawtmtivbiltage deliyeifed by' it'rjansfornif -TS OI TE reS'b ti'vefy, has'ndefie ct'bn 'boteritiai 6f the 'grld ofQVM Ede to the l ah ipirig atiiirilofjti ie diedes. A

W hr i aih eig fitirj ai'lger jluls e sapglied to tel mihaf Cl (Fig-5)the'si ip'p or ggflid Qf-VS gees besi tiv arid the siippfessqr Bias" is'emg'veg. n d rre thee l w se sc 'lfii z i fir-11s, e fi f s fal' al' iwe i e'i e 5 a P i ie'en in tn i Th a in an??? is ina smit e h hqoedeese q i trol i l and athq wr i t an ete By thi tim th h i ht :me kre w ee? bu the return h up r isse ari twe i ,9- tential olengfer guts'Qfi anode c irl ent since the iatltiqdelp'dtenti al has {11:0 peel. I i

is that a mas s a Wave iei v 3 ye d a the ath gear a reeii f s (:Figzza' d 2 n ti w- "Th st app d 1 1 s f a. idio'dEiS @I thereiqx efcarrjedpcgsitiye, a d the ano es tame i e it yefasd al are v iq .211 ,(iut-dff. The gifids .Qf andvt gare' then Free 0. move an rad a span i ,iiet re'b d 9p @13 :Q hed m tuh RP he a t hhs war Wei 59 (th iqethed sai'diss 'e bf th i jimta e 1 ,ve ye' V91 1 55 iansedtblcumqfixlurinst yeq g thfe resi e time base 'so'that the grids f t"g;e ,parapha se'aiik,n f ersvm -V1. arel v amps g sa preparaitgrylto .the build pip o'f a fuirtherlch g eltase -.to ,t' fpetwo BC and 136 T, du el ien f said ani-phase squar v aves 11 v 79.62".- mihed by QthefiHpfipglistarjtpf Q5313sizic e'the b i m l c5 i lioi i d s ha e thrsmeh a t enti l betenti ie ilandxeradu l ys a s s th i tsn ia b j ont e erm e ts d etu s i pr Q l fd.bia c ts e th twe ju enu vwi lg m ..r i.

ll A 11 .aircraft.radassystegn ini yqhieh the tel:- rai -b'en a .atairer f is S ann d rom? fieewel, a rotating directiveaerialdeviee; agath- ,ode raytube, a time base generatolicflnheeted tp [said aerialdevice, .syne hreniziriig means eon i eeted to said aerial device ar jd's a id tim i g'eneratdr; lockingme ansbQnnectdT-to said ube andgenerating recurrent voltage lelei iiiig of ti e same-recurrencefretguexlcy transmitted by said ti ansmitter reive? each frontsubstantiallycoincident with the in .stant of return of thefirstechesignaifrom said terrain, and icontroLmeans connected to and11eiIeaSinQsaid locking means-for initiating a.:cadial time-base sweepby each .of .said vrecurrent yeltage loeking fronts. V a

2 An aireyafturadar system comprising,-means {for irradiating theterrainbeheath th 'airfaft with an explol'i l g beam'ofpuljse-hodfilated radio ene1 g'y,'a"first cathode my tube, means fordefiecti'ng tfie elt ron beam of said first eath'o'de ray tube-by eehosignals x ec'eiyedin response to the l transmitted -pu1ses in amanrier'- to ihdicate the -range of the objects giving yis to said echosignals; means-for 'ge'neratfiglg atrain c Sf marker pulses each 'pulsebf Whichis I synchronized with 8 a transmitted pulse and is delayed withrespect thret'd; fnearis 'ftir"deffbtijagthe'lecti'on eastsaid'firsteatfiode'ray tube by said market pulses, means fbr'adjusting-'me'deiay betweeh'th'e trah's fn itted 'isul'ses' and the'maT-ke'r pulses tob'iirf'th e markerpu lse on the" screen "of the firsteathbd'e ray tub into coiiicidergcew'ith th'ec'hd" sigiiafd shortestfangefa second cathode ray ti1b,'meahs for prijdujemg' rotat'i'fig'TadiaI' tim-Base sweets did th'r'een or "said etups cathodei'ay'tuk'i,meafis' 61 intensity friqdiilating"saiHTadiaI t'iifibase swee s'withsaid eeiiosighals;anci'niafisfir iijitia'tiri g each 'of saidradial"time basesw 3 altitude, a iiuls .me ul tedtreee ittr- Number2,409,456 as-laces 2g422l204

